High-molecular weight polyester resins comprising a polybasic acid(s) and a polyhydric alcohol(s) (i.e., so-called oil-free alkyd resins) have been extensively used not only as fiber, film, and molding materials, but also as binders in the fields of paint, ink, adhesives and coatings. This is because of the satisfactory properties of these high-molecular weight polyester resins in dispersing pigments and in providing a coating film having excellent processability, chemical resistance, weather resistance and adhesion to various substrates. It is well known that the processability, water resistance, chemical resistance and weather resistance of a coating film formed of the high-molecular weight polyester resin can be improved without impairing the other physical properties of the film by employing aromatic polybasic acids, especially terephthalic acid, as an acid component of the polyester.
However, as the amount of aromatic polybasic acids (especially terephthalic acid) as a proportion of the total acid components increases, the polyester resin becomes less soluble in common organic solvents when the concentration of the resin is high, and therefore cannot be shaped in a fluid state. This severely limits the applicability of the resin. Thus, the development of a technique of fluidizing such resins in high concentration has been desired for a long time.
From the viewpoint of environmental and resource conservation, hazardous substance regulations, and improvement in the working environment, there has recently been an appreciable tendency to replace the organic solvent liquidization with an aqueous medium liquidization which means dissolving or dispersing a resin in an aqueous medium in order to be shaped. The same applies to polyester resins. In particular, an aqueous dispersion of a high-molecular weight polyester resin containing an aromatic polybasic acid component in a large proportion, especially a terephthalic acid component, is expected not only to form a coating film having excellent performance properties but also to exhibit excellent storage stability because of the hydrolysis resistance of this type of resin skeleton.
One method for dispersing a high-molecular weight polyester resin in aqueous medium comprises emulsifying a resin by dissolving in an organic solvent or melting, adding the solution or molten resin to an aqueous medium while imparting a high-speed shear to finely divide the resin, and stabilizing a dispersion of the resin with the aid of a dispersion stabilizer, such as a surfactant (called a forced emulsification method, hereinafter referred to as method A). Since the resin itself is hydrophobic, this method requires the use of a large quantity of a low-molecular weight hydrophilic surfactant in order to stabilize the dispersed fine particles. As a result, the coating film formed of the resin dispersion thus obtained has inferior water resistance. On the other hand, JP-B-59-30186 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-60-1334, JP-B-61-58092, JP-B-62-19789, JP-B-62-21380, and JP-B-62-21381 disclose a water-borne polyester resin, which is characterized by using a monomer having a metal sulfonate group (e.g., 5-sodium sulfoisophthalic acid) to introduce a metal sulfonate group which is a powerful hydrophilic group, into the polyester resin (method B). According to this method, since the hydrophilic group is incorporated into the resin, the resin can be easily dissolved or dispersed in an aqueous medium even though the content of the hydrophilic group is small. However, the above ionic group remains in the coating film after drying. This makes the film less resistant to water, corrosion and chemicals.
Yet another method for dispersing a polyester resin in an aqueous medium comprises synthesizing a polyester resin having a high acid value, neutralizing the carboxyl groups of the resin with a volatile basic compound, such as an organic amine compound, to form ionic hydrophilic groups in the resin (method C). Since the basic compound used in this method volatilizes while the coating film is dried, the resulting coating film has excellent water resistance, corrosion resistance and chemical resistance.
Method (C) includes (1) a method of dissolving the resin in an organic solvent or melting the resin, and adding the resin to an aqueous medium while imparting a shear force to obtain a fine dispersion in which the formed fine resin particles are stabilized by electric repulsion of the neutral salt to thereby prevent flocculation of fine resin particles (self-emulsification method), (2) a method of dissolving a resin in an organic solvent and pouring an aqueous medium into the solution while stirring to induce phase inversion from a W/O emulsion to an O/W emulsion, to thereby prepare a stable aqueous dispersion (phase inversion method), and (3) a method of mechanically grinding a polyester resin into fine particles and homogeneously dispersing the particles in an aqueous medium (slurry paint or aqueous slurry paint). As a modification of method (1), a method (4) is also known in which the liquidized resin of method (1) is finely divided by atomizing into an aqueous medium under high pressure.
However, each of the above-mentioned methods are problematic when applied to a high-molecular weight polyester resin containing an aromatic polybasic acid component (especially, a terephthalic acid component) in a large proportion. Thus, it has heretofore been impossible to obtain an aqueous dispersion in which the fine particles of such a polyester resin are stably dispersed in an aqueous medium in a high concentration. This type of polyester resin is not soluble in common organic solvents at a high solid concentration. Accordingly, an aqueous dispersion thereof, even if obtained by any of methods (1), (2) and (4) above, should contain a large amount of an organic solvent which deviates from what is essentially required from a water liquidization technique. On the other hand, JP-A-60-248734 and JP-A-60-248735 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") disclose a method in which a resin is dissolved in a specific organic solvent having a boiling point of 100.degree. C. or lower, and a part or all of the organic solvent is then removed from the O/W emulsion obtained by the method (2). However, the present inventors found that it is impossible to stably dissolve the polyester resin in such a specific organic solvent in a concentration of 50% by weight or higher even when accompanied by refluxing. Furthermore, the step of solvent removal requires not only a considerable amount of time and energy, but also a large quantity of a hydrophilic compound, such as a surfactant, so as to prevent flocculation of the resin particles during the solvent removal step.
With respect to the method of directly contacting a molten polyester resin with an aqueous medium, a danger is involved in emulsifying. This is because a molten polyester resin generally does not have sufficient flowability unless it is heated to 200.degree. C. or higher. Additionally, the polyester resin may markedly hydrolyze upon contact with the aqueous medium at high temperatures.
With regard to method (3), although various improvements have been proposed for securing dispersion stability of the resin particles, it is difficult to obtain fine particles having an average particle size of not greater than 1 .mu.m by mechanical grinding. Namely, this results in resin particles having a broad particle size distribution.
Therefore, the resulting aqueous dispersion does not have sufficient storage stability. Moreover, a coating film obtained therefrom has poor gloss, and a thin film cannot be formed.